Emergency department (ED) crowding has become a major public health crisis over the past decade. ED crowding is fundamentally a patient safety issue, and delays associated with ED crowding may adversely impact the care of patients with acute illnesses. However, policymakers and health system leaders have failed to address ED crowding as a top public health priority. The primary reason for inaction is the paucity of data demonstrating the impact of ED crowding on patient-level safety outcomes, such as deaths in the ED, deaths after ED discharge, and deaths after hospital admission. Without such evidence, policymakers are unlikely to commit substantial resources to address the root causes of ED crowding. To assess the impact of ED crowding on patient safety, investigators will perform a retrospective cohort study of all 2007 ED visits within Kaiser Permanente Southern California (KPSC), which is a regional integrated health system. This analysis will include 650,000 ED visits by KPSC member patients to 11 KPSC hospitals. ED crowding associated with each encounter will be measured using visit-specific exposures to crowding (waiting, treatment, and boarding times). This study has the following Specific Aims: 1. Assess the relationship between emergency department crowding and death in the emergency department. 2. Assess the relationship between emergency department crowding and outpatient outcomes after emergency department discharge, including 7-day bounce back hospitalization and death. 3. Assess the relationship between emergency department crowding and outcomes after hospitalization, including length-of-stay, in-hospital death, and 30-day death. The findings from this application will be used to design future studies of ED patient safety. The long- term goal of this research program is to design and evaluate health systems interventions to mitigate the impacts of ED crowding on patient safety. PUBLIC HEALTH RELEVANCE: Emergency department crowding is a nationwide phenomenon affecting the acute care system. This application will assess the impact of visit-level exposures of emergency department crowding on patient safety outcomes, including death in the emergency department, inpatient, and outpatient settings.